1. Field of the Invention
This invention pertains generally to systems and devices for tracking error signal generation and tracking error correction associated with writing and reading of optical data storage media. More specifically, the invention is a tracking error signal generation apparatus and method which utilizes confocal detection with a split detector and a differencing circuit for generation of tracking error signals.
2. Background
Various systems and methods for tracking error correction are known and used for writing and readout of optical data storage media. As optical media structures have become more complex and higher storage densities are achieved, improved tracking error correction systems are needed. There is accordingly a need for an improved tracking error signal generation apparatus and method. The present invention satisfies this need, as well as others, and generally overcomes the deficiencies found in the background art.
The invention is a tracking error signal generation apparatus and method which utilizes confocal detection with a split detector and a differencing circuit for generation of tracking error signals. In its most general terms, the invention comprises a first lens positioned in the path of a light beam reflected from (or transmitted through) an optical medium, a pinhole positioned in the path of the light beam proximate to the focal plane of the first lens, a second lens positioned in the path of the light beam after the pinhole, a split detector, having equal halves, positioned in the path of the light beam after the second lens, and a differencing circuit operatively coupled to each of the halves of the split detector. The reflected light beam will generally comprise a read beam on a return path from the optical medium. The second lens is positioned so that the focused beam emerging from the pinhole is re-imaged onto the split detector.
The invention is used with optical media having track patterns having differing reflectivity (or transmissivity) from the surrounding portions of the optical media, typically with the track having a higher reflectivity. In operation, a light beam is focused by an optical head onto a track in the optical medium, and the reflection of the beam from the optical medium is directed through the first lens, through the pinhole, through the second lens, and onto the split detector. When the focus of the light beam is centered on the track, the reflected light reaching the split detector is symmetrically and evenly distributed to the two halves of the detector, such that the differencing circuit will generate a tracking error signal (TES) having nominally a zero value. When the focus of the light beam is off-center, the reflected light received by detector is asymmertically distributed on the two halves of the detector such that the differencing circuit generates a non-zero tracking error signal, which will be of positive or negative value depending upon the direction off-center of the light beam focus spot. The tracking error signals thus generated are communicated to a servo system which will reposition the optical head to maintain the light beam focus spot on the center of the track.
The invention is particularly well suited for use with optical data storage media wherein tracks are defined by format grating structures. In one preferred embodiment, the optical media include a format hologram grating structure having vertical strips with high contrast carrier fringes separated by vertical strips with low contrast carrier fringes such that the high contrast carrier fringes define tracks. For disk shaped media the tracks are concentric and extend circumferentially around the medium to define cylindrical shells. Tracking is carried out by focusing the read spot on the center of the cylindrical shell or track.